Removing organic impurities from copper-cyanide electroplating baths



Patented Jan. 6, 1948 REMOVING ORGANI COPPER-CYANI BATHS C IIVIPURITIESFROM DE ELECTROPLATING Harry L. Benner and Robert R. Bair, NiagaraFalls, N. Y., assignors to E. I. du Pont de Nemours & Company, porationof Delaware Wilmington, Del., a cor- No Drawing. Application February 2,1942, Serial No. 429,285

6 Claims. (01. 204-52) This invention relates to the purification ofelectroplating baths and more particularly to the removal of organicimpurities from alkaline plating baths.

When electroplating metals from alkaline plating baths, for example,cyanide plating baths, the baths commonly become contaminated withorganic materials from various sources. Commcn sources of contaminationare lacquers and varnishes used to insulate plating racks or otherequipment or parts of the work plated, organic bufiing compounds, oils,greases and the like which have not been completely removed from thework prior to plating, and various accidental contaminations. Also insome plating baths organic materials are added as brightening orantipitting agents and these addition agents tend to gradually decomposeto form organic residues in the baths. The presence of such organiccontaminants has various adverse effects on the plating operation,depending upon the nature of the organic compounds present and otherfactors. In many cases it causes the formation of unsightly stains onthe electrodeposit, particularly in bright electroplating processes. Ifthe organic compounds are present as suspended solids, they tend tocause pits and roughness in the electrodeposited metal.

An object of the present invention is an improved method for thepuriflation of electroplating baths. A further object is a method forremoving organic contaminants from alkaline electroplating baths.Another object is to electrodeposit bright metal, bright copper, forexample, from alkaline plating baths. Other objects will be apparentfrom the ensuing description of the invention,

We have discovered that organic compounds can be efficiently andcompletely removed from an alkaline electroplating bath or renderedinnocuous by adding to the bath a soluble salt of an alkaline earthmetal so as to cause precipitation of an alkaline earth metal hydroxide.For the purpose of this invention, the term alkaline earth metal in thespecification and in the appended claims includes magnesium, as well ascalcium, barium and strontium.

Any soluble salt of an alkaline earth metal may be utilized inpracticing the invention. While effective results sometimes can beobtained by using soluble alkaline earth metal salts of organic acids,e. g. formates, acetates, propionates or butyrates, we generally preferto utilize the soluble inorganic salts. Examples of these are thesoluble alkaline earth metal chlorides, bromides,

iodides, nitrates, cyanides, thiocyanates, and nitrites, and magnesiumsulfate. Ordinarily, because of their cheapness and availability weprofor to use alkaline earth metal chlorides or magnesium sulfate.

The amount of alkaline earth metal salt required to be added will dependupon the nature of the electroplating bath and the alkaline earth metalsalt employed and upon the amount and kind of organic impurities to beremoved. We have found however that a very slight precipitate of analkaline earth metal hydroxide ordinarily is sufiicient to completelypurify a bath which is badly contaminated with organic impurities. Indetermining the amount of alkaline earth metal salt to be added, accountmust also be taken of the amount of carbonate ions in the bath, if any,and also the nature of the alkaline earth metal salt utilized. Forexample, calcium carbonate is more insoluble than calcium hydroxide; andhence, in order to precipitate the desired calcium hydroxide in theelectroplating bath, we have found it necessary first to add sufiicientcalcium salt or other carbonate precipitant to complete theprecipitation of any carbonate ions as calcium carbonate and then to adda further amount of the calcium salt to cause subsequent precipitationof the desired amount of calcium hydroxide. Alternatively, the carbonateions first may be wholly or partially removed by any other suitablemeans before precipitating the calcium hydroxide. If the carbonate ionsare removed by precipitation with the calcium salt, it is not essentialthat the carbonate precipitate be removed before adding the remainder ofthe precipitate of the calcium to calcium hydroxide, The sameconsiderations apply to the use of salts of barium and strontium toprecipitate the corresponding hydroxides.

On the other hand, magnesium hydroxide is more insoluble than magnesiumcarbonate and hence magnesium hydroxide may be precipitated in a bathcontaining carbonate ions without recipitating any substantial amount ofcarbonate and without first removing the carbonate ions. For thisreason, unless it is desired simultaneously to remove carbonate from thebath, we generally prefer to use a magnesium salt for the bathpurification, if carbonate is present. Hence we prefer to use a solublemagnesium salt to treat cyanide baths, which usually contain appreciableamounts of carbonate.

The amount of alkaline earth metal hydroxide ordinarily required forcomplete bath purification is small. In most cases a precipitate of thehydroxide equal to 0.1 to 0.3 per cent by weight of the electroplatingbath will be sufiicient although in some cases it may be necessary toprecipitate a larger amount of the hydroxide.

The means by which the hydroxide precipitate removes the organicimpurities has not been determined. This phenomenon apparently cannot beexplained simply as the resultof precipitating an inorganic materialbecause we have found that the precipitation of aluminum hydroxide,which sometimes is used to purify water, isnot suitable for purificationof an alkaline plating bath. Also, as indicated above, we have'foundthat the precipitation of carbonate without precipitation of thehydroxide does not give the desired results.

The precipitate of alkaline earth hydroxide may or may not be removedfrom the bath before resumption of electroplating, as desired. When theamount of precipitate is large it is sometimes desirable to remove itfrom the bath, as the particles of precipitate may tend to cause roughdeposits by coming in contact with the cathode, In many cases however,where the amount of precipitation required is slight, the bath may besatisfactorily operated without removal of theprecipitate.

In practicing our invention, we may stop operation of a contaminatedelectroplating bath and precipitate therein the alkaline earth metalhydroxide as above described. This may be done in the electroplatingtank, or the bath may be transferred to another vessel, as desired. The

.bath then may be again placed in operation, with or without firstremoving the hydroxide precipitate. If the amount of precipitate issufficiently small, or where solid particles in the bath are notobjectionable, the bath purification may be carried out withoutinterrupting the electroplating operations. However, we prefer to purifythe bath without electrolysis, either by stopping the flow of current orby transferring all or part of the bath to a separate vessel forthepurifying treatment.

In one preferred modification, for example, we may continuouslyorintermittently circulate the bath through a purifying vessel where thealkaline earth metal salt is added and the resulting hydroxideprecipitate is removed by filtration or settling and the purifiedelectrolyte led back to the plating tank. Various other modes ofoperating our invention will be apparent to those skilled inelectroplating.

The following examples illustrate specific modes of practicing ourinvention:

Example I A copper cyanide plating bathwas made up containingapproximately:

Oz. per gal. Copper cyanide 8 Free cyanide (NaCN) 0.5 Sodium hydroxide 4Sodium carbonate 3.8

droxide, deposits were again bright at a current density of 30 amps. persq. ft.

Example II =A bright'copper cyanide plating bath in satisfactoryoperation to produce bright plate contained:

This bath-was contaminated by adding small -,.amounts of v.buiiingcompound, oil and grease, .boiled for fourhours and filtered. The baththen was placed in operation and the resulting copper plate was stainedand mottled, exhibiting the I typical efiects of organic contaminants.

The bath then was treated by adding 8 oz. per gal. of calcium chloride(CaC12), which was an excess of approximately 1.73 oz. per gal. overthat required to precipitate all carbonate in the bath. The resultingprecipitate of calcium carbonate and hydroxide was filtered off and thebath replaced in operation. Good bright plate was obtained from thepurified bath in a current density range of 10 to amps. per sq. ft.

Example III An operating conventional bright zinc bath, which producedsatisfactory bright zinc plate contained:

Oz. per gal. Zinc cyanide 10 Sodium cyanide '5 Sodium hydroxide 10 andfiltering off the precipitated magnesium hydroxide, the bath producedgood bright plate at the aforesaid current densities.

Our method is applicable to all alkaline electroplating baths, whereinthe degree of alkalinity is sufficient to cause the desiredprecipitation of alkaline earth metal hydroxide. Thus, for example, ourprocess is adapted for the purification of the various cyanide baths forplating metals such as zinc, copper, brass, cadmium, silver, gold andbronzes (tin-copper alloys) and also for other alkaline plating baths,e. g., for plating zinc and. tin (stannate and stannite baths). If thealkalinity is not sufiiciently great to precipitate the required amountof alkaline earth metal hydroxide, we may add sufficient alkalinereagent such as an alkali metal hydroxide or an alkali metal salt, e.g., sodium or potassium cyanide, tetraborate, carbonate or the like,which will impart .the required degree of alkalinity to the bath. .Inmostalkaline plating. solutions, such addition of alkali is unnecessary.

We claim:

1. The process for removing deleterious organic compounds from a usedelectroplating bath containing hydroxyl ions and carbonate ions whichcomprisesprecipitating magnesium hydroxide in said bath by adding asoluble magnesium salt' to said bath and replacing the acidradicles ofsaid magensiumsalt with hydroxyl radicles substantially withoutprecipitating magnesium carbonate therein.

2. The'process for removingdeleterious organic compounds from a usedcopper cyanide electroplating bath containing hydroxyl ions andcarbonate ions which comprises precipitating magnesium hydroxide in saidbath by adding a soluble magnesium salt to said bath and replacing theacid radicles of said magnesium salt with bydroxyl radiclessubstantially without precipitating magnesium carbonate therein.

3. The process for removing organic impurities from an alkalineelectroplating bath containing carbonate ions and sufiicient hydroxylions to replace the acid radicles of a magnesium salt which comprisesadding thereto a soluble magnesium salt in amount sufiicient toprecipitate magnesium hydroxide but substantially insufficient toprecipitate magnesium carbonate therein.

4. The process for purifying a cyanide electroplating bath containingcarbonate ions and sufficient hydroxyl ions to replace the acid radiclesof a magnesium salt which comprises adding thereto a soluble magnesiumsalt in amount sufi'icient to precipitate magnesium hydroxide butsubstantially insufiicient to precipitate magnesium carhonate.

5. The process for removing organic impurities from a copper cyanideelectroplating bath containing carbonate ions and sufllcient hydroxylions to replace the acid radicles of a magnesium salt which comprisesadding thereto a soluble magnesium salt in amount sufficient toprecipitate magnesium hydroxide in an amount equal to about 0.1 to 0.3%of the weight of said bath, substantially without precipitatingmagnesium carbonate therein.

6. In a process for electroplating bright copper from a metal cyanidebath normally containing carbonate ions and sufificient hydroXyl ions toreplace the acid radicles of a magnesium salt and subject tocontamination with organic impurities which cause imperfections in theelectrodeposit, the step comprising adding to said bath a solublemagnesium salt in the amount required to precipitate therein an amountof magnesium hydroxide equal to about 0.1 to 0.3% of the weight of thebath, substantially without precipitating magnesium carbonate therein.

HARRY L. BENNER. ROBERT R. BAIR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,164,924 Hull July 4, 19392,243,696 Ferm May 27, 1941 OTHER REFERENCES Blum & Hogaboom, Principlesof Electroplat-ing and Electroforming, 2d edition (1930), published byMcGraw-Hill Book Co., New York, page 227.

Transactions of the Electrochemical Society, vol. 80 (1941), page 364.

